1. Field of the Invention
The present invention relates to sealing compositions and, in particular, to compositions for sealing alumina ceramic packages in which integrated circuits or other solid state devices are encapsulated.
2. Prior Art
A known composition for sealing alumina ceramic packages comprises a mixture of vitreous older glass powder, lead titanate powder, and zircon (zirconium silicate, ZrSiO.sub.4) powder, as disclosed in Japanese Patent Publication No. 56-49861. This known sealing composition has good mechanical strength and resistance to thermal shock. However, it has high dielectric constant and therefore, is not good to seal packages containing large-scaled integrated circuits such as memories. Further, the composition disadvantageously radiates .alpha.-rays because zircon usually available includes radioactive impurities such as uranium and/or thorium therein whereby soft errors are caused in integrated circuits of DRAM and CCD types.
U.S. Pat. Nos. 4,405,722 and 4,421,947 disclose sealing compositions including zircon which is not desired by the above-described reason.
U.S. Pat. No. 4,405,722 discloses a mixture of vitreous glass powder and cordierite powder including .beta.-eucryptite, .beta.-spodumene, zircon, and/or lead titanate as optional ingredients. The sealing composition, even if zircon is not used, is not high in mechanical strength, and has a tendency to suffer from micro-cracks caused by heat shock resulting in damage to the hermetical seal.
U.S. Pat. No. 4,421,947 discloses a sealing composition having crystalline filler other than zircon, but the sealing composition is not a powdery mixture and has a high sealing temperature of about 1225.degree. F.
Another known sealing composition comprises a mixture of PbO-B.sub.2 O.sub.3 or PbO-B.sub.2 O.sub.3 -ZnO solder glass powder and non-inert zinc zirconium silicate powder as disclosed in U.S. Pat. No. 3,963,505. However, the sealing composition is not so desirable because zinc zirconium silicate included therein includes radioactive impurities such as uranium and/or thorium.
U.S. Pat. No. 4,621,064 by Matsuura et al proposes to use synthetic zircon in place of natural zircon in a sealing composition. That is, the proposed sealing composition is a mixture of 50-80 wt % vitreous solder glass powder of PbO-B.sub.2 O.sub.3 system having a deformation point of 350.degree. or less, 1-35 wt % zinc material powder, and 1-35 wt % synthetic zircon powder artificially prepared to eliminate radioactive impurities such as uranium and/or thorium.
The sealing composition using the synthetic zircon radiates no .alpha.-ray, but the composition has problems of flowability degraded and thermal expansion coefficient increased by inclusion of the synthetic zircon increased.
The present invention found out that the problems were caused by presence of non-reacted zirconium oxide (ZrO.sub.2) in the synthetic zircon.
The synthetic zircon is obtained by reacting purified silica (SiO.sub.2) sand and purified zirconium oxide (ZrO.sub.2) using a reaction accelerator such as NaF, NaCl, and/or Na.sub.2 SO.sub.4. However, the obtained synthetic zircon includes non-reacted zirconium oxide.
Further, the synthetic zircon includes the reaction accelerator NaF, NaCl and/or Na.sub.2 SO.sub.4. It is also disadvantageous that the synthetic zircon includes alkali such as Na because the sealing composition using the synthetic zircon is degraded in electric insulation.